A plastic LCD design with high reliability and color-free readability

A new design has been developed for a plastic substrate twisted nematic LCD. The design results in color-free optical performance and high reliability as measured by accelerated life tests at high temperature and humidity. The crystalline nature of most chemically stable plastics has a biaxial birefringence which can produce dramatic interface colors in the twisted nematic geometry. Optically isotropic plastics are generally amorphous and, therefore, chemically incompatible with the liquid crystal which is a good organic solvent. This dichotomy has been solved by using a special form of polyester whose optic axes are controlled to lie at an angle from the normal which is considerably larger than the largest angle which can be reached by light entering the high index plastic. A fabrication process was developed to be consistent with the 150°C glass transition temperature for the polyester as opposed to the 500°C transition temperature of typical LCD glass substrates. Display materials were designed to be water tolerant, since all plastics transmit water. It was established on a laboratory scale that a plastic LCD design will pass consumer type reliability criteria for high-temperature and humidity stresses. Displays as large as 50 in
²
were fabricated and preliminary operational tests were encouraging. The plastic LCD process has been shown to be a technologically feasible manufacturing method and may hold promise for significant cost reduction over glass technology.